Conventionally, the spectrum licensed by mobile carriers for cellular networks (e.g., Long-Term Evolution, LTE) and the unlicensed spectrum used for wireless local-area networks (WLANs) have been largely independent. As the number of mobile nodes and the data rates per mobile node increase, there has been a growing interest in smaller cells transmitting LTE channels in bands (or carriers) of the unlicensed spectrum.
Examples of unlicensed spectrum bands are the commonly-called “WiFi bands” around 2.4 GHz, 5 GHz and 60 GHz. These are free to use provided that certain regulatory constraints are fulfilled when transmitting on them. These bands are not dedicated to a specific radio-access technology (RAT).
Licensed-Assisted Access via LTE (LAA-LTE) has recently been proposed for LTE co-existence on unlicensed carriers with, e.g., WLAN. This means that a user equipment (UE) is connected to an LTE network on a licensed spectrum band, the so-called primary cell (P-cell). In addition, it can also be connected to the same network on an unlicensed spectrum, the secondary cell (S-cell).
Data transmission on an unlicensed band is to be controlled by a Listen-Before-Talk (LBT) mechanism, at least is some geographical regions. For example, an LBT mechanism may not be required in the United States in certain situations. Standards, e.g., future 3GPP standards for LAA-LTE, may require that an LBT mechanism is applicable to all regions. The LBT mechanism applies a functionality assessing the band to be clear before the transmission begins. This is necessary to avoid collisions, e.g. between WLAN and LTE networks, since there is no frame structure and no centralized synchronization governing the transmission in WLAN networks. Hence, any LTE transmitter must first listen to the channel for a stipulated time for energy detection before commencing transmission.
However, the first LTE node that “grabs” the channel blocks other LTE nodes in the neighborhood performing channel sensing.